In general, an austenite-based stainless steel known that its workability and corrosion resistance are excellent contains Cr and Ni as main raw materials, using iron (Fe) as a basis metal, and has been developed as various steels suitable for various kinds of uses by adding other elements such as Mo and Cu. The austenite-based stainless steel is a steel having excellent corrosion resistance and pitting corrosion resistance, and contains low carbon and Ni of 8% or more in weight percentage (wt %). However, in case of Ni, the range of fluctuation in price, caused by an increase of cost, is large, and therefore, price competitiveness is lowered. Accordingly, in order to solve such a problem, many studies have been conducted to develop a duplex stainless steel which contains a low content of Ni and has corrosion resistance equal to or greater than that of the austenite-based stainless steel.
The duplex stainless steel is a steel in which each volume fraction of austenite phase and ferrite phase is 35 to 65%. The duplex stainless steel has a low content of Ni while ensuring corrosion resistance equal to that of the conventional austenite-based stainless steel, and hence is economical. Further, it is easy to ensure high strength, and hence the duplex stainless steel has come into the spotlight as a steel material for industrial facilities including fresh water, pulp, paper making and chemical facilities, which require corrosion resistance. In addition, among duplex stainless steels, interest in a lean duplex stainless steel which further increases the advantage of a low alloy cost by excluding high-priced alloy elements such as Ni and Mo and adding low-priced alloy elements in place of the high-priced alloy elements has recently been increased. The low-alloy duplex stainless steel is usually referred to as a lean duplex stainless steel. Hereinafter, it will be explained that “lean low-alloy duplex” and “lean duplex” have the same meaning.
However, in case of the low-alloy duplex stainless steel, it is important that when the content of Ni and Mo is reduced, excellent corrosion resistance is ensured by controlling the balance of the austenite and ferrite phases due to the reduction in the content of Ni and Mo. In addition, it is important to improve hot workability capable of suppressing a defect or the like which may occur in manufacturing a plate of the lean duplex stainless steel. Generally, the lean duplex stainless steel has a low content of Cr, Mo and Ni, as compared with the existing duplex stainless steel, and therefore, the corrosion resistance of the lean duplex stainless is decreased. In addition, the stability of the ferrite and austenite phases is decreased due to the decrease in content of the alloy elements, and therefore, it is difficult to control the balance of each phase. Further, the phase fraction is rapidly changed depending on annealing temperature, and therefore, it is difficult to ensure an appropriate corrosion resistance level. Further, when welding is performed, the corrosion resistance of a heat affected zone (HAZ) portion may be lowered, and hence it is important to sufficiently ensure the corrosion resistance of a base metal.
In case of the duplex stainless steel, a defect occurs at surface and edge portions of the plate in hot deformation due to microstructure characteristics of the ferrite and austenite phases. The occurrence of such a defect becomes serious in the lean duplex stainless steel in which the content of the alloy elements is decreased. Ordinarily, it is known through several experiments that the temperature at which the hot workability of the duplex stainless steel is weakest is 900° C. The hot workability of the lean duplex stainless steel is weak in a low-temperature region of 800 to 900° C. When a material is hot-rolled, the temperature of the surface of the material is lowered to the temperature region due to contact of the material with a low-temperature roll when the roll is contacted with the surface of the roll, and therefore, a defect easily occurs in surface and edge portions of the material. Accordingly, it is necessary to improve hot workability in the temperature region.
For these reasons, the lean duplex stainless steel is limitedly used when the corrosion resistance of the lean duplex stainless steel is not problematic so much even though the alloy cost of the lean duplex stainless steel is considerably low. Therefore, there is a limitation in widely using the lean duplex stainless steel as a substitute of the existing austenite-based stainless steel. Further, since the lean duplex stainless steel is produced using a Steckel mill which easily secures the temperature of a hot-rolled plate even in production of the plate due to inferior hot workability, the lean duplex stainless steel is disadvantageous in terms of cost and productivity, as compared with that produced using a tandem mill.
Accordingly, an object of the present invention is to provide a lean or lean duplex stainless steel which can decrease the content of Cr, Mo and Ni, and ensure an appropriate corrosion resistance equal to or greater than that of 304 or 304L stainless steel that is an austenite-based stainless steel.
Another object of the present invention is to provide a lean or lean duplex stainless steel which can ensure excellent hot workability capable of suppressing a plate edge cracking defect and a surface cracking defect in the manufacturing of a plate of the lean or lean duplex stainless steel.